The human airway represents a unique microenvironment where a delicate immunologic balance is required to provide protection from foreign substances without disrupting the vital function of the lung. Gamma/delta T cells are a prominent feature of the airway mucosa of animals where they are believed to act as sentinels of the immune system that sense epithelial stress and perform immunoregulatory functions. In humans, much less is known about gamma/delta T cells in the airway mucosa, which has been difficult to study for practical reasons. While limited studies have shown that lung gamma/delta T cells levels are elevated in certain human diseases, their role in normal health and disease pathogenesis remains undetermined. We have recently reported the generation and characterization of gamma/delta T cell lines from human airway biopsies, identified several different stimuli capable of evoking human gamma/delta T cell responses in vitro and used expression profiling to identify genes that may be important to their in vivo effector functions. Our preliminary data also suggests that T cell receptor (TCR)-mediated selection occurs among human airway gamma/delta T cells. Based on these studies and data reported in the literature, we hypothesize that (A) gamma/delta T cells in the human airway mucosa play a vital role in the host response to inhaled antigens or injury and are activated in a non MHC-dependent manner via their TCR, possibly by host "stress" antigens, (B) the repertoire of gamma/delta T cells in the human airway mucosa is restricted as a reflection of TCR-mediated clonal selection to locally encountered antigens, and (C) gamma/delta T cells play an important role in regulating lung immune homeostasis through their effects on epithelial and alpha/beta T cells in their microenvironment. To address these hypotheses, we propose to pursue in vitro studies with human samples, rather than use animal models, since previously reported studies have demonstrated that important differences exist between human gamma/delta T cells and those of other species, especially with regard to antigens and TCRs. We propose to (1) generate and characterize gamma/delta T cell clones from human airway biopsies to determine the mechanisms crucial to airway gamma/delta T cell growth, including antigenic stimuli, requirements for antigen processing and MHC restriction, (2) identify dominant g/d T cell populations in human airway mucosa, and (3) determine the effects of airway gamma/delta T cells on airway epithelial and alpha/beta T cell populations. The results of these experiments should provide important insights into the role of airway mucosal gamma/delta T cells in maintaining human pulmonary immune homeostasis.